1. Field of the Invention
The present invention relates generally to a support structure for tensely extending and supporting a flexible film or sheet while tensely extending the film or sheet. More specifically, the invention relates to a tensely extending support structure for tensely extending and supporting a film or sheet for a projection screen of a projection television or the like.
2. Description of the Prior Art
As conventional Fresnel lenses for projection screens of televisions and so forth, rigid relatively-thick sheets have been used. As such Fresnel lenses, circular Fresnel lenses having a plurality of concentric lenses are often used. Such circular Fresnel lenses are formed by the thermal press or by means of a mold using an ultraviolet curing resin.
In recent years, there is a demand for decreasing the thickness of a lens sheet in order to prevent the picture quality of a lens from deteriorating due to the optical loss of the lens, and there is also a demand for the continuous production in order to improve the productivity, so that lenses formed on a thin film have been produced.
For example, a linear Fresnel lens, which is formed by arranging linear lenses in parallel, is made by means of a machine, to which a gravure printing press is applied, since it has a linear shape. That is, an ultraviolet curing resin is transferred to a continuous thin film so as to have a predetermined shape, and the ultraviolet curing resin is cured by ultraviolet rays to form a linear Fresnel lens (the ultraviolet curing method).
In a case where a flexible thin film with lenses thus produced is used for a projection screen, there are known two methods for tensely extending and supporting the film. In one of the methods, the film is combined with a rigid plate, e.g., a lenticular lens sheet. In the other method, the film itself is tensely supported on a support, such as a frame and a cabinet. In order to increase the productivity by decreasing the probability of generation of wrinkles and sags and in order to decrease the thickness of the film, the latter method is preferred.
However, in the case of the latter method for tensely supporting the film with lenses on the support, it is difficult to surely secure the film to the support. It is also difficult to continue to exert tension on the film with lenses so as not to produce wrinkles therein, so that there is a need for the know-how of fixing of the film to the support. That is, in the latter method, it is required to always exert tension on the film with lenses, and it is also required that the whole film with lenses can be simply held so as not to produce wrinkles and sags.
As described above, one would wish to provide a simple tensely extending support structure, which can surely secure a flexible thin film with linear Fresnel lenses continuously produced for a projection screen to a support, such as a frame and a cabinet, which can continue to exert tension on the film without producing wrinkles and sags, and which does not need the know-how of fixing.